1. Field of the Invention
The present invention is generally in the field of electrical circuits. More particularly, the invention is in the field of voltage controlled oscillators.
2. Description of the Related Art
Voltage controlled oscillators (“VCOs”), such as VCOs used in wireless communications devices, typically require a wide tuning range to cover desired frequency bands and channel bandwidths as well as frequency alterations due to process variations. VCOs that employ LC tank circuits typically use varactors for continuous frequency tuning. However, using large varactors for continuous tuning over a wide frequency range can cause a large VCO gain, which undesirably increases sensitivity to noise and disturbances on the control voltage. To solve this problem, a small varactor can be used in combination with a switched capacitor array to achieve digital tuning (i.e. coarse tuning) and continuous tuning (i.e. fine tuning). Either digital tuning or continuous tuning requires a closed loop to implement a desirable auto-tuning function.
Current digital auto-tuning approaches however, have a number of significant drawbacks. One conventional approach uses a separate high speed divider with a fixed division ratio for VCO digital auto-tuning. However, in this approach, the fixed division ratio of the high speed divider reduces resolution and the separate high speed divider increases semiconductor die size and cost. Another conventional VCO digital auto-tuning approach utilizes an analog frequency comparator, which is difficult to design with high resolution and low cost. Other conventional VCO digital auto-tuning approaches have undesirable features such long tuning time and/or limited resolution.
Thus, there is a need in the art for a low cost, high resolution digital auto-tuning circuit for a VCO.